Attention has recently been focused on GaN-based compound semiconductor light emitting devices as semiconductor materials for short wavelength light emitting devices. GaN-based compound semiconductors are formed by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) and the like on a substrate consisting of sapphire single crystal, various oxides or a group III-V compound.
The external quantum efficiency of a light emitting device is expressed as the product of light extraction efficiency and internal quantum efficiency. Internal quantum efficiency refers to the proportion of electrical current energy injected into a light emitting device that is converted to light. On the other hand, light extraction efficiency refers to the proportion of the total amount of light generated within a semiconductor crystal that can be extracted to the outside.
The internal quantum efficiency of light emitting devices has recently been improved to about 70 to 80% by improving the quality of gallium nitride-based compound semiconductor crystals and by examining the structure of the emission layer, and adequate effects have been obtained relative to the amount of injected current.
However, in not only the case of GaN-based compound semiconductors, but also in the case of light emitting diodes (LED), the light extraction efficiency relative to generated light is generally low, and it is difficult to say that internally emitted light generated by injected current is adequately extracted to the outside.
The cause of low light extraction efficiency is that light emitted in an emission layer is repeatedly reflected and absorbed by the crystal material within the LED structure, and when that light is radiated outside the semiconductor layer, emitted light is not extracted to the outside due to reflection beyond the critical angle according to Snell's law, thereby resulting in the light being re-reflected within the crystal and lowering the probability of light reaching the outside.
A technology has been proposed for improving this light extraction efficiency in which the light extraction surface is modified and the angle of the light extraction surface is changed in various ways to improve the light extraction efficiency (see, for example, Japanese Unexamined Patent Publication No. 2003-110136 and Japanese Unexamined Patent Publication No. 2004-6662).
In addition, the inventors of the present invention studied the use of an inverted tapered shape for the semiconductor layer on the lateral surface of a light emitting device that has an angle of less than 90 degrees relative to the substrate main surface for the purpose of enabling light to be efficiently extracted from the upper surface of a light emitting device. However, when a gallium nitride-based compound semiconductor light emitting device having this inverted tapered lateral surface was mounted on a typical light emitting package, and the relationship between the emission angle and output characteristics was investigated, although a satisfactory emission output was obtained in the direction of the front surface due to light reflected by the inverted tapered lateral surface, adequate emission output was unable to be obtained where the emission angle is low, namely in the direction of the lateral surface of the light emitting diode, thereby demonstrating that light distribution uniformity is not satisfactory.
There has recently been a growing demand among LED light emitting packages for light emitting packages having a lower height than conventionally used light emitting devices for the purpose of reducing space and improving efficiency. In particular, modifications have been made to the package portion for mounting light emitting devices in which the interval between the package lateral surface and the light emitting device has been reduced in order to improve overall package efficiency by utilizing reflection of light. In such cases, technology for controlling and utilizing the light extraction angle from the device is important. A light emitting device having an inverted tapered lateral surface as described above has been experimentally determined to not allow the obtaining of adequate device characteristics in these special light emitting packages utilizing light from the lateral surface of semiconductor light emitting device.